Time Assignment Speech Interpolation (TASI) communication systems have been known for approximately twenty years. Their function, stated in general terms, is to exploit the fact that during a normal telephone conversation, information is being transmitted only about 35% of the time. TASI communication systems serve to connect a speaker to a transmission line only during those portions of a conversation when speech is actually present. During the other portions of the conversation, the transmission line is connected to another speaker, currently speaking in another conversation.
By using time assignment speech interpolation a given number of transmission lines, say 24 can usually carry about 48 simultaneous conversations.
ASI systems are extensively represented in the patent literature. Some examples of U.S. patents in this area are listed below: Nos.
______________________________________ 4,138,597 3,878,337 3,508,007 2,927,968 4,131,765 3,864,524 3,466,398 2,907,829 4,124,777 3,848,093 3,424,869 2,870,260 4,100,377 3,840,708 3,424,868 2,692,303 4,095,052 3,836,719 3,406,257 2,651,678 4,066,844 3,832,491 3,311,707 2,631,194 4,061,878 3,811,014 3,304,373 2,629,021 4,059,730 3,804,990 3,304,372 2,629,020 4,048,447 3,803,405 3,153,196 2,564,419 4,052,068 3,790,715 3,118,018 2,548,661 4,032,719 3,721,767 3,056,858 2,541,932 4,012,595 3,712,959 3,046,347 2,388,001 4,005,276 3,706,091 3,042,752 2,311,707 4,002,841 3,686,442 3,030,447 2,301,223 4,001,505 3,681,533 2,974,198 2,271,000 3,997,729 3,668,645 2,961,492 1,905,359 3,985,956 3,660,645 2,958,733 1,873,786 3,927,286 3,649,766 2,957,946 1,852,727 3,890,467 3,644,680 2,957,945 4,147,896 3,882,458 3,560,660 2,948,779 U.S. Pat. RE 23,313 3,520,999 2,941,039 Application RE 25,546 3,510,596 2,935,569 863,902 ______________________________________
Most of the prior art patents known to applicants and listed above relate to systems in which assignment, synchronization and acknowledgement messages between TASI apparatus on opposite ends of a transmission trunk link are passed along control lines, separate from the voice carrying lines, This arrangement involves a number of disadvantages, principal among which is the vulnerability of the entire transmission trunk link to failure of the control lines, Another disadvantage is the inability of the conventional TASI systems to readily sense and bypass inoperative voice lines.
U.S. Pat. No. 4,147,896 described TASI apparatus in which messages are transmitted along voice channels by means of a parallel combination of tones. This apparatus suffers from the disadvantage that the number of message bits is severely limited thereby limiting the message vocabulary. The apparatus has limited reliability due to the use of tone transmission.
A number of the above-listed patents describe TASI-type apparatus for use with a T-1 digital trunk. U.S. Pat. No. 4,066,844 discloses such a system in which the assignment information is included in some of the 24 digital time slots employed in voice channels. This patent and U.S. Pat. Nos. 4,048,447 and 3,330,605 which exemplify such systems are limited in their application to operation with digital trunk lines and are inherently incapable of operating with an analog telephone trunk.
Although the use of digital telephone trunks is increasing rapidly in the U.S., the majority of telephone trunks in the U.S. and throughout the world will remain analog during the coming few decades. There is therefore a need for TASI-type equipment capable of use with both analog and digital telephone trunks. There is also need for apparatus capable of operation with analog trunks which overcomes the design disadvantages of the prior art analog apparatus which is described in many of the above-listed patents. No such apparatus presently exists.
It is also noted that no TASI-type equipment is presently available for use with out of band signalling.